452 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
but since the work of Boveri, Hertwig brothers, Roux, Drieseh, Wilson, 
Morgan, Loeb, and others on the fragments of eggs, the development 
of embryos, abnormal and normal, from portions of eggs is a question 
no longer to be doubted. 
Formation of the Ectoderm. 
In the development of the egg of Turritopsis the germinal layers are 
not differentiated by the process of epibole, delamination, or cellular 
digression. During segmentation the blastomeres do not separate 
and arrange themselves around a segmentation cavity which later is 
transformed into a blastocoele. Thus instead of having formed a 
coeloblastula, we find that cleavage results in the formation of a solid 
oval embryo destitute of a blastocoele, which may be called a morula 
stage. The cells of the segmenting egg are all alike in structure and 
nearly equal in size; so that they are not distinguishable into primitive 
ectoderm and primitive endoderm, which is the case in forms where a 
definite delamination takes place, as is so beautifully shown in Liriope 
and Geryonia, and in species where cellular digression occurs as in 
Stomotoca and Clytia for example. Figures 34 (pi. 33) to 39 (pi. 34) 
illustrate the uniformity of the cells, and the solid character of the egg 
during segmentation. In figure 36 (pi. 34) a space exists between 
the blastomeres near one end of the egg, but this is not to be regarded 
as a true cleavage cavity. The next figure shows three of these false 
cleavage cavities. They occur only occasionally. As stated before 
most of the eggs are entirely solid. 
About the time the irregular mass of segmenting blastomeres is 
metamorphosed into the oval embryo, the cell boundaries are lost for 
a short time and a syncytium is formed. This syncytial structure is 
crowded with yolk granules and nuclei are scattered throughout the 
protoplasm. The nuclei soon become more numerous near the peri¬ 
phery; and then cell walls begin to appear as shown in plate 34, 
figure 47. These cells are to become the ectoderm, which is soon 
separated from the inner structureless mass by the development of 
the mesogloea. Now the ectoderm forms a distinct layer, composed 
of columnar cells all of which are at first similar in structure and lie 
parallel to each other as shown in figure 48 (pi. 34). The differ¬ 
entiation of the ectoderm cells takes place later. 
